Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as energy sources. Among these secondary batteries, lithium secondary batteries having high energy density and voltage, long life span and low self-discharge are commercially available and widely used.
The lithium secondary batteries generally use a carbon material as an anode active material. Also, the use of lithium metals, sulfur compounds, silicon compounds, tin compounds and the like as the anode active material are considered. Meanwhile, the lithium secondary batteries generally use lithium cobalt composite oxide (LiCoO2) as a cathode active material. Also, the use of lithium-manganese composite oxides such as LiMnO2 having a layered crystal structure and LiMn2O4 having a spinel crystal structure and lithium nickel composite oxide (LiNiO2) as the cathode active material has been considered.
LiCoO2 is currently used owing to superior physical properties such as cycle life, but has disadvantages of low stability and high-cost due to use of cobalt, which suffers from natural resource limitations, and limitations of mass-use as a power source for electric automobiles. LiNiO2 is unsuitable for practical application to mass-production at a reasonable cost due to many features associated with preparation methods thereof. Lithium manganese oxides such as LiMnO2 and LiMn2O4 have a disadvantage of short cycle life.
In recent years, methods to use lithium transition metal phosphate as a cathode active material have been researched. Lithium transition metal phosphate is largely divided into LixM2(PO4)3 having a NASICON structure and LiMPO4 having an olivine structure, and is found to exhibit superior high-temperature stability, as compared to conventional LiCoO2. To date, Li3V2(PO4)3 is the most widely known NASICON structure compound, and LiFePO4 and Li(Mn, Fe)PO4 are the most widely known olivine structure compounds.
Among olivine structure compounds, LiFePO4 has a high voltage of 3.5 V and a high bulk density of 3.6 g/cm3, as compared to lithium, has a theoretical capacity of 170 mAh/g and exhibits superior high-temperature stability, as compared to cobalt (Co), and utilizes cheap Fe, thus being highly applicable as the cathode active material for lithium secondary batteries.
However, LiFePO4 exhibits low electrical conductivity, thus disadvantageously causing an increase in inner resistance of batteries, when used as the cathode active material. This increase also leads to an increase in polarization potential, when electric circuits close, and thus a decrease in battery capacity.
In this regard, prior arts including Japanese Patent Application Publication No. 2001-110414, etc. disclose incorporation of a conductive material into olivine-type metal phosphate to improve conductivity.
However, LiFePO4 is generally prepared by a solid-phase method or a hydrothermal method using Li2CO3 or LiOH as a lithium source. These methods have a disadvantage in that a large volume of Li2CO3 is produced during baking due to lithium and carbon sources added to improve electrical conductivity.
Such Li2CO3 may be decomposed upon charge or react with an electrolytic solution to produce CO2 gas, thus disadvantageously generating excessive amount of gas during storage or cycles. This also disadvantageously causes a swelling phenomenon and deterioration in high-temperature stability.
Accordingly, there is an increasing need for lithium iron phosphate such as LiFePO4 that exhibits superior electrical conductivity, while containing a minimum amount of Li2CO3.